Power amplifier time-delay invariant predistortion methods and apparatus

ABSTRACT

An embodiment of the invention is a time-delay invariant predistortion approach to linearize power amplifiers in wireless RF transmitters. The predistortion architecture is based on the stored-compensation or memory-compensation principle by using a combined time-delay addressing method, and therefore, the architecture has an intrinsic, self-calibrating time-delay compensation function. The predistortion architecture only uses a lookup table to conduct both the correction of non-linear responses of a power amplifier and the compensation of any time-delay effects presented in the same system. Due to the time-delay invariant characteristic, the predistortion design has a wider dynamic range processing advantage for wireless RF signals, and therefore can be implemented in multi-carrier and multi-channel wireless systems.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/724,157, filed Dec. 21, 2012 which is acontinuation application of U.S. patent application Ser. No. 12/021,241,filed Jan. 28, 2008, titled, “Power Amplifier Time-Delay InvariantPredistortion Methods and Apparatus,” which in turn is acontinuation-in-part of U.S. patent application Ser. No. 11/262,079,filed Oct. 27, 2005, titled “System and Method for Digital MemorizedPredistortion for Wireless Communication” and also claims the benefit ofU.S. Provisional Patent Application No. 60/897,746, filed Jan. 26, 2007,titled “Power Amplifier Time-Delay Invariant Predistortion Methods andApparatus,” and U.S. Provisional Patent Application No. 60/898,312,filed Jan. 29, 2007, titled “Power Amplifier Time-Delay InvariantPredistortion Methods and Apparatus,” the continuation-in-part Ser. No.11/262,079 of which in turn is a continuation of U.S. patent applicationSer. No. 10/137,556, filed May 1, 2002, now U.S. Pat. No. 6,985,704,issued Jan. 10, 2006 titled “System and Method for Digital MemorizedPredistortion for Wireless Communication,” all of which are incorporatedby reference in their entirety for all purposes. This application alsoincorporates by reference U.S. patent application Ser. No. 11/799,239,filed Apr. 30, 2007 and its parent U.S. Provisional Patent ApplicationNo. 60/795,820, filed Apr. 28, 2006, titled “High EfficiencyLinearization Power Amplifier For Wireless Communication”; related nonprovisional U.S. patent application Ser. No. 11/962,025, filed Dec. 20,2007, titled “Power Amplifier Predistortion Methods and Apparatus;” andits parent U.S. Provisional Patent Application No. 60/876,640, filedDec. 22, 2006, titled “Power Amplifier Predistortion Methods andApparatus.”

FIELD OF THE INVENTION

The present invention relates systems and methods for usingpredistortion to linearize the output of a power amplifier (PA), such asthose used in wireless transmission systems. More particularly, thepresent invention uses an adaptive time-delay adjusting methodology tocorrect the PA's nonlinearity. More particularly, the invention relatesto systems and methods which use a time-delay invariant predistortionarchitecture to linearize the output of a power amplifier (PA) inwireless transmission systems.

BACKGROUND OF THE INVENTION

In a typical prior art predistortion system for the linearization ofpower amplifiers, the system typically contains multiple signaltransmission paths, such as a reference path and a feedback path. In apredistortion linearization system, when a signal passes throughdifferent signal transmission paths, such as a reference path and afeedback path, the occurrence of a timing difference caused by thedistinct signal paths is inevitable. This difference, usually referredto as a time delay, presents significant problems with respect to theaccuracy of the predistortion correction. These problems are made worseby the fact that the time delay can vary with environmental conditionsincluding temperature, system conditions including signal power level,system aging, and so on. Therefore, it is difficult to measure in alaboratory such an intrinsic time delay parameter, and, moreover, aparameter for correcting the time-delay cannot be designed as aconstant. Many efforts have been made in the prior art to compensatefor, reduce or eliminate this time-delay. Traditionally, two methodshave been used in prior art predistortion circuits to solve the problemcreated by the time-delay.

The first is to make a special time-delay cable by measuring andcalculating the difference of the same signal passing through differenttransmission paths so as to compensate the time-delay effect, such asthe processing in analog feed-forward predistortion systems. Thisapproach suffers the limitation that the time-delay cable imposes afixed time-delay that cannot be adjusted despite changes in the signaland environment which naturally occur during the operation of practicalsystems.

The second is to use a special digital signal processing (DSP) algorithmand circuitry to calculate and adjust adaptively the time difference ofthe same signal passing through different transmission paths, and thenuse the resulting time-delay information to correct the non-linearity ofpower amplifier. This approach is usually implemented in a digitalfeedback approach and a wireless environment. However, an extra circuit,typically a latch, and an associated algorithm are needed, and theaccuracy of the time-delay calculation is also related to theconvergence rate of the algorithm.

SUMMARY OF THE INVENTION

Next-generation wireless communication systems will demand improvedtransmitted signal quality and improved overall RF transmitter systemperformance for a variety of broadband and multimedia services. Thesedemands upon advanced RF transmitter systems will be satisfied, at leastin part, by power amplifiers with higher power efficiency and higherspectrum efficiency than is currently available. In order to get abetter predistortion result, the present invention evaluates thetime-delay parameter as a variable that is estimated and calculated by aspecial algorithm and circuitry.

More particularly, the present invention uses, in one implementation,one combined predistortion and time-delay lookup table structure toprovide correction factors for both the PA's non-linear distortion andthe system's time-delay. This permits the system and method of thepresent invention to be a self-calibrating solution for performanceimprovement and nonlinear correction in wireless RF transmitter systems.

This design can be conveniently implemented by an elegantly simplecircuit structure and can be used for almost all wireless radiofrequency (RF) transmission systems to improve both power efficiency andspectrum efficiency. Examples of some applicable RF transmission systemsinclude wireless base-stations, access points, mobile handsets includingbut not limited to cellular and GPRS protocols, mobile wirelessterminals, portable wireless devices, and other wireless communicationsystems such as microwave and satellite communications.

The new time-delay invariant method presented herein uses a combinationof (i) an adaptive time-delay adjusting methodology to process the PA'snonlinearity, and, at the same time, (ii) time-delay compensationwithout the additional circuitry and/or an algorithm tailored for thespecial time-delay adjustment. This novel algorithm-based methodologycan be implemented by a predistortion processing unit that includes atime-delay addressing lookup table that can store and memorize the PA'snon-linearity, time-delay information and other interference in thesystem such as noise.

These and other features of the present invention can be betterappreciated from the following detailed description, taken inconjunction with the accompanying Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system and apparatus in accordance with the presentinvention.

FIG. 2 illustrates in simplified form one implementation for serialaddressing of the lookup table as well as the entry of the accumulatedhistory into the lookup table.

FIG. 3 illustrates in simplified form an implementation of paralleladdressing of the lookup table, which is otherwise as shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

In one implementation of the invention, the entries for the lookup tableare developed in accordance with the techniques described in pendingU.S. patent application Ser. No. 11/262,079, filed Oct. 27, 2005, andU.S. patent application Ser. No. 11/799,239, filed Apr. 30, 2007, bothof which are incorporated herein by reference. It will be appreciated bythose skilled in the art that the range of errors which occur in areal-world system are bounded; that is, there is a range which has aminimum value and a maximum value, and in all but rare circumstances thecorrection factor applicable at the time of any given sample will fallwithin that range. By selecting a lookup table of appropriate size, andpopulating the lookup table with appropriate values as determined by themethods in the above-mentioned patent, which are selected over theentire operating spectrum of the PA and associated system, thecorrection factor which is appropriate for each sampling of the inputsignal will be one of the values already in the lookup table. Thus, oncethe lookup table has been fully populated, the correction factors of thepresent invention do not vary with time; that is, they aretime-invariant. Although the size of the lookup table can varysignificantly with the particular implementation, and could for somesystems be as small as sixteen entries, for more complex implementationssuch as those appropriate for wireless RF transmission systems the tablewill have on the order of 2¹² entries or more, and may havesignificantly more entries depending on the permissible powerconsumption, cost, and related system factors. For certainimplementations, a table size between 2¹² and 2¹⁴ entries has been foundto be acceptable.

In one implementation, the lookup table of the predistortion processingunit, or predistorter, is addressed by a set of time-related addressesthat can be structured by a shift register, although parallel addressingmay be implemented in some schemes, as discussed in greater detailhereinafter. For present purposes, the shift register technique will beused for illustration. The addressing of the lookup table is based upona stored-compensation or memory compensation principle that storesinformation at different time by a vector form and maps the input vectorinto one of the entries in the lookup table. The set of addressedentries in the lookup table will result in an output signal that is amapping function of the corresponding input vector. Due to the inputaddress vector comprising different time signals, the output signal ofthe lookup table is actually related to the different time information,including the current signal and previous N transmitted signals, whereN>1 and N is an integer. As a result, the signal stored in each entry ofthe lookup table can be considered as a combination of all pasttransmitted signals rather than the sole response of the current inputsignal. Typically, the bit length of the address vector in the lookuptable determines the duration of time-delay signal to be covered.

In order to correct for the PA's non-linearity in the time-delayenvironment with a lookup table unit according to the invention, withoutthe use of a latch or other secondary correction circuit, thepredistortion algorithm utilizes a function to incorporate thetime-delay signal combination. The lookup table stores the non-linearinformation derived from the PA together with a time-delay factor thatis caused by the different signal transmission paths. By appropriatelyaddressing the lookup table in response to a sample, the lookup tableprovides a correction factor which includes both the appropriatepredistortion correction and the appropriate time delay compensation.The output of the lookup table is then combined with the original inputsignal to provide an input to the PA that results in a linearizedoutput, with substantially no time delay error.

Due to the memory and store function of the lookup table, the non-linearcharacteristics of the PA that are to be corrected by the lookup tableare not limited by the time-based data. The time-independent feature ofthe lookup table's adaptive processing is one benefit of at least someimplementations of the addressing arrangement of the lookup table. Theaddressing of the lookup table is implemented by a set of N-bit vectordata that comprises the current input signal as well as the previous Ninput signals. Therefore, the address of the lookup table is acombination of series of input sequences with the length of N. Thelonger the address of the lookup table (and therefore the larger thelookup table), the wider the range of time-delay information that thesystem can accommodate, i.e. the longer duration of time-delay effectsthat the system can tolerate. However, while a larger table may permitstorage of more information, at some point the entries becomerepetitive, such that a larger table offers diminishing marginalreturns, wastes memory resources, and unnecessarily increases powerconsumption.

The lookup table of the predistortion processor is based upon thestored-compensation principle that maps a set of input vectors to a realsignal output. Since the address of lookup table contains the inputinformation stored from different time, each output signal generated bythe lookup table is closely related to the transmitted multi-signalscombination. Therefore, based upon the arrangement of the lookup table,the table update entries are also closely related to the combinationinformation of input signals stored from different time points.

Referring next to FIG. 1, an embodiment of the present invention may beappreciated in greater detail. More particularly, the illustratedembodiment includes an Analog Multiplier 11 which receives a modulatedRF signal V_(RF) from the RF modulator portion 10 of the base station,and also receives a predistortion correction signal Vp from a lookuptable and related components which can be generally described as apredistortion processor, discussed in greater detail below. In general,the predistortion processor can be thought of as all of the componentsbetween the ADC's 21 and 25 and the DAC 30. The output of the analogmultiplier 11 is provided as the input V_(in) to the power amplifier(PA) 12, which in turn transmits an output signal V_(o) to an Antenna13. The RF modulator 10 is typically although not necessarily aquadrature modulator or an orthogonal modulator. It will be appreciatedthat multiplier 11 can be implemented as multiple multipliers, eachassociated with one or more quadrature signals.

An input down-converter circuit 20 receives an idealized referencesignal V_(RF) from modulator in base station, and is biased by a localoscillator 40, such that it provides an output V_(d) to ananalog-to-digital converter 21. The ADC 21 converts the signal V_(d) todigital form (as I and Q signals), whereupon it is provided as one inputpair to the Digital Predistortion Processor, and more specifically tovariables 221 and 22Q, respectively.

A feedback down-converter circuit 26, also biased by a local oscillator40, receives a raw feedback signal V_(o)(t) from the output of the PA,and provides a feedback signal V_(f) to an feedback ADC 25. The digitaloutput of the ADC 25 then provides a second input, i.e. feedback signal,to the Digital Predistortion Processor, and more particularly tovariables 241 and 240. The Digital Predistortion, discussed in greaterdetail below, provides a digital output signal V_(r) to a DAC 30, whichconverts the digital signal to an analog form, where it is combined withthe modulated RF signal in the multiplier 11.

As shown in FIG. 1, address data formers 32I-32Q receive inputs from theADC 21 I/Q, and are designed to generate the required signal format fora lookup table 33 I/Q. The data formers 32 I/Q address memory unitswithin the lookup tables 33 I/Q, where the lookup table providesseparate I and Q outputs to an adder 31. It will be appreciated that thelookup table 33 can be implemented as one or more lookup tables. Theaddress provided by the address formers 32I-32Q can be considered alookup-table key or address.

The predistortion controller lookup tables 33I-33Q are designed memoryunits to store the predistortion signal for high power amplifierlinearization. The predistortion signals in the tables are based on theerror generated by a comparison of the ideal signal v_(d) and thefeedback signal v_(r) and the presented adaptive algorithm. The datastored in the tables 33 I/Q can be updated by adaptive iteration asdescribed hereinafter, and forms digitally indexed data reflecting thenonlinear characteristics of the power amplifier.

By comparison of AM-AM and AM-PM information between the idealizedsignal V_(RF)(t) and the feedback signal V_(o)(t), the DigitalPredistortion Processor calculates the error in the amplitude and phasecomponents of the output signal V_(o)(t) caused by the non-lineartransmission characteristics of the high power amplifier 12.

Based on the error information obtained by the foregoing comparison, thepredistortion processor, based on the lookup table algorithm disclosedin U.S. Pat. No. 6,985,704, calculates and generates adaptively acompensation signal that is of inverse characteristics with thetransform function of the PA 12 to pre-distort the AM-AM and AM-PMdistortion caused by the PA 12.

The outputs v_(p) of the predistortion lookup table 33I-33Q are fed tomultiplier 11, after an adder 31 and a digit-to-analog converter 30, tomodify the modulated RF signal from modulator 10. The output of themultiplier is the required predistortion signal V_(in)(k) that is of aninverse non-linearity with that of the power amplifier 12 to yield apre-compensation to the input of high power amplifier.

It will be appreciated by those skilled in the art that there can be asignal difference between two signals, ideal signal V_(RF) and feedbacksignal, V_(o)(t), when they arrive at the predistortion controller. Thetime difference results from the time-delay differences between the twosignals, which is caused by the different paths each travels in arrivingat the controller. This signal time-delay can vary randomly based on theparameters of the circuits and parts, as well as other environmentalfactors. The result is that it is difficult to estimate, calculate andadjust for such signal differences in the field application environment.To overcome this issue, the present invention adaptively adjusts forthis time-delay through the use of an algorithm taught by the previouslycited U.S. Pat. No. 6,985,704.

The use of the look-up tables 33 permits a memory function to beintroduced into at least some embodiments of the present invention. Thelookup table of the predistortion controller is based on a storedcompensation principle that maps a set of input data into a digitaloutput, and updated adaptively. Based on the stored function, eachoutput signal of lookup table is actually related to both the currentand the previously transmitted signal, and therefore has a memoryfunction which compensate not only for the non-linearity of the PA, butalso avoids the need for a special time-delay compensation circuit suchas typically used in the prior art.

Based on the predistortion architecture shown in FIG. 1, thenon-linearity of power amplifier is corrected by the output signal v_(p)of predistortion processor. The signal v_(p) is multiplied with themodulated RF signal to generate the predistorted signal as the input ofpower amplifier. Actually, the input signal of power amplifier is acomplex gain signal that can be controllable in the amplitude/envelopeand be adjustable in phase. The relation of the input and output can bedescribed as the following complex gain expressionv _(in) =v _(RF) v _(p) =v _(RF) F(V)  (1)where v_(p) is the output of predistortion processor generated by amapping function F of lookup table. Usually, the mapping function F isunknown and is difficult to express mathematically. However, F may bedetermined adaptively by updating the entries in the lookup table inaccordance with the adaptive algorithm to realize all possible mappingthat corresponds to the relations with{0,1}^(N) →v _(p).

The lookup table therefore maps each set of N-bit input address vector Vto a real output v_(p). In fact, the N-dimensional address vectorrepresents the transmitted signal sequences going through poweramplifier from the current time though previous N time, expressed byV(k)=(d ₁(k),d ₂(k), . . . ,d _(N)(k))^(T)  (2)where each data d_(i) in above vector V is either 1 or 0, expressed asd _(i)(k)=0 or 1 for 1≤i≤N  (3)

In one implementation, shown in FIG. 2, the address of the lookup table200 is formed by a serial shift register 205 that addresses eachcorresponding entry of the lookup table during the predistortionprocesses. Due to the addressed information being related to the currentand previous N transmitted signal, the output signal 210 of the lookuptable can be considered as a function of the last N transmitted data,and therefore incorporates a time delay correction element. The lookuptable is adaptively updated by combining the adaptive error 215 and atable update 220 in combiner 225. The arrangement of the lookup table,configured as a predistortion processor, can systematically utilize thecombined correction signal for the processing of both non-linearitycorrection of PA and time-delay compensation, even if time-delay effectsare present when the same signal passes through different transmissionpaths.

The technique of using a lookup table having combined predistortion andtime-delay correction makes unnecessary the use of additional time-delayprocessing and related circuitries. Since the address of the lookuptable contains the current and previous transmitted correctioninformation, each output signal from the lookup table is inherently amulti-time information combination that contains rich signal componentsfrom the current time to the previous N-time. As a result, thepredistortion algorithm has a built-in mechanism to compensate forsignal-delay effects. This results in a predistortion processingstructure that is significantly simpler, and more effective thantraditional solutions.

In a predistortion architecture with the lookup table as shown in FIGS.1 and 2, the memory table is addressed by its address register. Thebit-number length of shift register determines the size of lookup tableand therefore the covered time-delay range, i.e., the maximum limit oftime-delay effects. Each set of data stored in a lookup table entry hasa unique address index. This address index of a data set corresponds tothe data set's sampled time point of the input signal. The addressindices are then utilized as time stamps for time calibration purpose ofPA's non-linear error correction calculations. In other words, at eachpredistortion calculation time point, the predistortion algorithmselects only one set of data from a specific addressed entry as theoutput of the lookup table for further PA's non-linearity errorcorrection processing. If the selected output signal is only related tothe current-time transmitted signal without any relation with othertransmitted signal, then the time-delay, caused by the currenttransmitted signal passing through different transmission paths, must beconsidered during the predistortion processing in order to provide anaccurate signal matching between the reference and feedback signal.

The time-delay range to be covered by lookup table is related to the bitnumber of the address register. For a lookup table with an N-bit addressregister, the size of lookup table entries is M=2^(N−1). This means thatthere are M entries in the lookup table, and all the data stored in theentries are the functions of address vectors that cover the current timek to previous k−M+1 time. The set of input address vectors, A, can beexpressed asA={V(k),V(k−1), . . . ,V(k−M+1)}  (4)where V(k) is an input address vector at time k, which records Mpossible predistorted information to PA nonlinearity and eachinformation recoded the current transmitted signal and the time-delaysignal components.

Based on the memory-compensation principle, the combined time-delaylookup table structure involves simple logic operations and signalprocessing to capture the time-delay of signal passing through differenttransmission paths. More specifically, due to the time-delay invariantcharacteristic, this predistortion architecture can process and correctPA non-linearities in a wider dynamic range and without the needs ofadditional circuitries and algorithms.

It will also be appreciated by those skilled in the art that, for someembodiments, typically those with larger lookup tables and relativelyhigh sampling rates, a parallel addressing scheme can be used, such asthat shown in FIG. 3, which is essentially identical to FIG. 2 exceptfor the use of a parallel input 300 to the lookup table. If the sampleddata points are taken close together (i.e., a comparatively fastsampling rate) so that the correction factors for sample t and samplet+1 effectively become identical, then we can use the paralleladdressing approach to address the lookup table. In some embodiments, ifthe size of the lookup table is sufficiently large, and computing powerand other device issues are satisfied, then the parallel addressingapproach can offer better performance over a serial addressing approachin environments which use a very fast sampling rate.

Having fully described the invention in detail in detail, includingseveral embodiments and alternatives, those skilled in the art willappreciate that numerous other alternatives and equivalents exist whichare within the scope of the present invention. Therefore the inventionis intended not to be limited by the above description, but rather onlyby the appended claims.

What is claimed is:
 1. A predistortion system for linearizing the outputof a power amplifier, the predistortion system comprising: a firstreceiver configured to receive a first signal representative of a radiofrequency (RF) modulated signal; a second receiver configured to receivea feedback signal representative of at least one nonlinearcharacteristic of the power amplifier; and a predistortion controllercommunicatively coupled with both the first receiver and the secondreceiver, wherein the predistortion controller comprises at least onelookup table of a predetermined size, and wherein an output of thelookup table includes a time delay correction element.
 2. Thepredistortion system of claim 1 wherein the inputs are further derivedfrom a difference between the square of a quadrature component of thefirst signal and the square of a quadrature component of the feedbacksignal.
 3. The predistortion system of claim 1 wherein the inputs arefurther derived from a difference between the square of an in-phasecomponent of the first signal and the square of an in-phase component ofthe feedback signal.
 4. The predistortion system of claim 1, furthercomprising a reference path communicatively coupled with the firstreceiver and the predistortion controller.
 5. The predistortion systemof claim 4, further comprising a feedback path communicatively coupledwith the second receiver and the predistortion controller.
 6. Thepredistortion system of claim 5 wherein the at least one lookup table isconfigured to store at least one time-delay value that compensates for atime delay between the reference path and the feedback path.
 7. Thepredistortion system of claim 6 wherein the at least one lookup table isfurther configured to adaptively update at least one correction value inthe lookup table by combining a non-linear correction value of the poweramplifier with the at least one time-delay value.
 8. The predistortionsystem of claim 1 wherein the predistortion controller is configured togenerate a correction factor for correcting at least one nonlinearcharacteristic of the power amplifier based on at least an input of thelookup table.
 9. The predistortion system of claim 8, further comprisinga processor configured to combine the RF modulated signal with a secondsignal corresponding to the correction factor.
 10. The predistortionsystem of claim 9 wherein the processor is further configured to supplythe combined RF modulated signal with the feedback signal to the poweramplifier to linearize the output of the power amplifier.
 11. Thepredistortion system of claim 8, further comprising combining logic tocombine the RF modulated signal with a signal corresponding to thecorrection factor.
 12. The predistortion system of claim 11 whereincombining logic is performed to also supply the combined RF modulatedsignal and feedback signal to the power amplifier to linearize theoutput of the power amplifier.
 13. The predistortion system of claim 1,further comprising a serial shift register for forming an address forthe lookup table.
 14. The predistortion system of claim 1 wherein thelookup table is addressed by parallel signals.
 15. The predistortionsystem of claim 1 wherein the feedback signal is an analog signal. 16.The predistortion system of claim 15, further comprising adigital-to-analog converter.
 17. The predistortion system of claim 16wherein the digital-to-analog converter is configured to convert adigital signal corresponding to a correction factor of the feedbacksignal.
 18. The predistortion system of claim 1 wherein the lookup tablecomprises at least one correction value.
 19. The predistortion system ofclaim 18 wherein the at least one correction value falls within a rangedetermined by the predetermined size of the lookup table.
 20. Thepredistortion system of claim 1 wherein the lookup table is configuredto respond to inputs derived from outputs of the at least one lookuptable.